Webmail issue

In our office server windows server 2008 std version, We have only one NIC, connected through D-Link Switch, i have been configure ip address manually.

Our company webmail URL only not working in our environment, outlook also not working. But out of the office its working fine.

When i remove our preferred DNS Server(My Office Server IP Adddress) from the client machine (Windows 7), at that time, that URL is working fine in our office environment. Kindly give me your opinions & also give me a solutions for that issues.